1. Field of the Invention
The present invention pertains to a system, including an array of geophones and a data recording and analysis system, and method for detecting "micro-earthquakes" caused by the extension of hydraulic fractures and a method for analyzing and displaying data related to the location of the fracture extension so that, for example, fracture extension resulting from injection of fluid or slurried solids waste materials into a formation zone through an injection well may be controlled.
2. Background
Hydraulic fracturing of certain zones of earth formations is a now commonly used method to stimulate the production of hydrocarbon fluids, for example. More recently, it has been proposed to dispose of certain fluids or slurried solids waste materials into certain zones of earth formations which will contain these materials by hydraulically fracturing the zone to a predetermined extent to at least partially provide space for disposal of such waste material. However, a long-standing problem in causing hydraulic fractures is determination of the lateral, as well as the vertical, extent or growth of the fracture away from the point of fracture origination, typically a fluid injection well. In particular, the geometry of hydraulic fractures created by the injection of waste materials into a formation zone of interest is critical to avoid causing the injected material to flow beyond predetermined boundaries and contaminate aquifers or other paths which may lead the waste material into unwanted areas. The growth or propagation of hydraulic fractures is also desirably measured to properly control the production of fluids from formation zones of interest.
An important consideration in monitoring the propagation of hydraulic fractures is determination of the fracture boundaries, including the radial extent of the fracture from an injection well on a so-called real-time basis so that, for example, the injection of liquid and slurried solids waste materials into an earth formation may be controlled to prevent deposition of the materials in unwanted zones. Certain governmental authorities have jurisdiction over what earth formations and zones of certain earth formations may receive waste materials. The accurate determination of the propagation of a hydraulic fracture on a real-time basis is thus of considerable importance.
Although the detection of seismic events such as micro-earthquakes and the like is a relatively developed art, there has been a need to provide a system and method for measuring certain micro-earthquake events, such as those caused by the propagation of hydraulic fractures, to separate the actual event indicating propagation of a fracture from other acoustic noise in the formation, such as fluid flow generated noise and surface transmitted noise from machinery and the like. Accordingly, the process of gathering large amounts of data generated during the extension of hydraulic fractures, and the rapid analysis and display of such data in a meaningful way which will indicate the location of an event which is part of a fracture propagation process, is of utmost importance to such endeavors as the disposal of slurried solids wastes and other fluid wastes through subterranean injection wells.
Moreover, heretofore there has been no direct ability to monitor the growth of hydraulically induced fractures during such growth, and the extent of fracture length, height, width and growth rate have been assumed from pre-fracture computations for a predetermined set of characteristics of the earth formation in which the fracture is induced. These fracture models have not always been sufficiently accurate and the ability to actually measure the lateral as well as vertical growth of a hydraulic fracture has been a long-sought goal. An article entitled "Active and Passive Imaging of Hydraulic Fractures" by P. B. Willis, et al, Geophysics: The Leading Edge of Exploration, July 1992, describes a system for monitoring the growth of hydraulic fractures from one or more instrumented monitor wells wherein the seismic events are measured by geophones and the resultant geophone signals are later subjected to signal analysis. An article entitled "The Application of High Frequency Seismic Monitoring Methods for the Mapping of Grout Injections" by E. L. Majer, The International Journal of Rock Mechanics, Mining Science and Geomechanics, Vol. 26, Nos. 3 and 4, pages 249-256, 1989, discusses a real-time monitoring system wherein digital data at sample rates greater than 50,000 samples per second are required to capture the necessary data. A sixteen channel system digitizes the data at up to 100,000 samples per second on each channel, saves the waveform, picks p-wave times, locates the events and plots the results in "close to real time using 3-D color graphics". The present invention provides an improved system and methods for determining the extent and location of hydraulic fractures propagated in the earth, particularly, but not limited to, those fractures caused by the injection of fluids through an injection well for waste disposal.